The invention relates to a picture display device comprising a picture display tube, a field deflection circuit for generating a deflection current having a trace and a retrace through a field deflection coil for the vertical deflection of one or more electron beam(s) generated in the picture display tube and a pulse generator for generating a signal for blanking the electron beam(s) at least in the field retrace period.
In such a picture display device, for example a television receiver, a pulse-shaped signal generated by the pulse generator is either applied directly to an electrode of the picture display tube or added to the video signal. This causes the electron beam(s) to be blanked in the field retrace period in which period the display screen of the tube does not light up, so that the lines occurring in this period are not visible.
The retrace period of the field deflection current, that is to say the period in which the deflection current varies rapidly from its maximum value into one direction to its maximum value into the other direction, depends, however, on the inductance value of the deflection coil and, consequently, on the tolerances thereof and on the voltage across the deflection coil and, consequently, any fluctuations thereof. So the retrace period is not known accurately. Mostly the pulse duration of the blanking signal is chosen to have such a long duration that the longest retrace period to be expected is still shorter. The consequence thereof is that not only the lines immediately occurring after the field synchronizing pulse are invisible, as desired, but also useful information at the beginning and/or at the end of the trace period, so at the top and/or at the bottom in the display picture, may be lost.